1. Field of the Invention
The present invention relates generally to an optical distance measuring arrangement, and more particularly pertains to an arrangement for interferometrically measuring the distance between two reflectors, at least one of which is movable with respect to the other. The present invention has particular utility in optical interferometric laser ranging systems which provide very precise incremental distance measurements, and are useful, for instance, in precisely measuring or mapping the contour of a precision optical surface such as a mirror.
2. Discussion of the Prior Art
Laser ranging systems are well known in the prior art, and generally involve two retroreflectors, one positioned on each of two points or surfaces, between which a precise distance measurement is desired. A laser beam is then reflected between the two retroreflectors such that any change in distance therebetween results in a change in the length of the optical path traversed by the laser beam between the retroreflectors. This change in optical path length can be precisely measured interferometrically by setting up an interference pattern between the transmitted laser beam and the returned laser beam as each change in the optical path length of one wavelength results in the interference pattern being modulated through one cycle.
U.S. patent application Ser. No. 282,552 for Measuring System With Redundant Arrangement of Measuring Points, by Greenleaf & Watson, commonly assigned herewith, discloses one application of the present invention in an arrangement for measuring the contour of a complex optical surface in which a redundant configuration of laser ranging distance measuring systems is positioned above the surface being measured. Each laser ranging system monitors changes in distance measurements to a point proximate the surface as the measuring point is moved across the surface. The distance measurements are taken from a sufficient number of points on the surface such that a redundant quantity of data is obtained to define both the system geometry and also the position of each measurement point. The data is then transformed by recognized mathematical techniques into the coordinate positions of all the measured points on the surface.
In the aforesaid patent application, the laser ranging distance measuring systems are differential distance measuring systems in the form of Laser Unequal Path Interferometers (LUPIs) arranged in a tetrahedral configuration above the measured surface, although in alternative embodiments the distance measuring systems could be absolute interferometers operating with several different frequencies of light, all of which can implement the teachings of the present invention. In the context of the particular technological field described in that patent application, the development of distance measuring arrangements providing surface measurements as accurately as possible is an essential part of a program to develop a system for accurately surfacing complex optical surfaces such as large aspherics.